Add ability to get the current working directory in dart script

[efortuna]

Currently, the only way I can find to determine what directory the user is in is by running
new Process.start('pwd') and adding a lineHandler to capture the output, which is not OS-independent and unclear whether it would even work since the Windows equivalent is echo %cd%, which requires running in the shell.

[DartBot]

This comment was originally written by ladicek@gmail.com

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I've got a slighly better one (tested on Linux only, but I bet it works elsewhere too):

  Directory cwd() => new Directory(new File(".").fullPathSync());

  var d = cwd();
  print(d.path);
  print(d.existsSync());

Not ideal, though. I'd love to see a Directory.current() constructor -- and there's even a bug for that, http://code.google.com/p/dart/issues/detail?id=1793

[efortuna]

Added Duplicate label.
Marked as being merged into #1793.

[kevmoo]

Removed Area-IO label.
Added Area-Library, Library-IO labels.